1. Field of the Invention
The present invention relates to a method for manufacturing semiconductor devices using a self-aligned fabrication process.
2. Background
Fabrication of an integrated circuit (xe2x80x9cICxe2x80x9d) comprises a sequence of processing steps to produce electrical devices contained in a semiconductor chip. During the production of ICs, various layers of thin films of dielectrics (e.g., SiO2, Si3N4, etc.), polysilicon and metal conductors (e.g., Al, Ni, Au, Pt, Ti, etc.) are grown or deposited on the surface of a semiconductor wafer (xe2x80x9cwaferxe2x80x9d), one on top of the other.
Deposition is an operation in which a film is placed on the wafer surface, usually without a chemical reaction with the underlying layer. Deposition is also known as xe2x80x9cevaporationxe2x80x9d. Two common techniques used for forming these thin films are physical vapor deposition and chemical vapor deposition. Chemical vapor deposition is a process in which insulating or conducting films are deposited on a substrate by use of reactant gasses and an energy source to produce a gas-phase chemical reaction. The energy source may be thermal, optical or plasma in nature. Physical vapor deposition is a process in which a conductive or insulting film is deposited on a wafer surface without the assistance of a chemical reaction. Examples of physical vapor deposition include vacuum evaporation and sputtering.
To perform its intended function, each deposited layer must be located within a specific region on the wafer surface. This is accomplished by forming a desired layer and then patterning and etching it to provide a certain device function in specific areas of the wafer. Often a mask is used in this patterning and etching process. Lithography is a transfer process where the pattern on the mask is replicated in a radiation-sensitive layer located below the mask on the wafer surface. Typically, this has been accomplished with UV light as the radiation source and photo-resist (xe2x80x9cresistxe2x80x9d), which is a UV-sensitive polymer, as the mask layer. First, a few drops of resist are deposited on a wafer which is spinning at a slow rate to produce a uniform coating and the spin speed is increased to enhance drying. The wafer with resist is softbaked at 80-90xc2x0 Centigrade for 10-30 minutes to drive off the remaining solvents. Next, the wafers are put in an exposure system and the mask pattern to be transferred is aligned to any existing wafer patterns. The resist is exposed through the mask to UV radiation that changes its structure, depending upon whether the resist type is positive or negative. For a positive type resist, the resist dissipates when exposed to light and a subsequent solvent application. In contrast, the negative type resist hardens after exposure to light and is not removed by the subsequent solvent application. The resist is not affected in regions where the mask is opaque in either case. After full wafer exposure the resist is developed such that the unpolymerized regions are selectively dissolved in an appropriate solvent. The polymerized portion of the resist remains intact on the wafer surface reflecting the opaque features of the mask in a positive resist and just the opposite for a negative resist.
After the resist pattern is formed it is then transferred to the surface layer of the wafer. Sometimes this is an invisible layer, such as ion implantation, but more often than not it is a physical transfer of the pattern by etching the surface layer, using the resist as a mask. This either results in the desired structure or produces a more etch-resistant mask for further pattern transfer operations.
Two common types of etching processes used in semiconductor fabrication include wet etching and dry etching. Wet etching is a process which uses liquid chemical reactions with unprotected regions of a wafer to remove specific layers of the substrate. During wet etching, wafers with resist (or a resist transferred mask) are immersed in a temperature-controlled etchant for a fixed period of time. The etch rate is dependent on the strength of the etchant, temperature and material being etched. Such chemical etches are isotopic, which means that the vertical and lateral etch rates are the same. Thus, the thicker the layer being etched, the more undercutting of the mask pattern. Most wet etches are stopped with an underlying etchstop layer that is impervious to the etchant used to remove the top layer.
Dry etching is also used during semiconductor fabrication. Dry etching is a process that uses gas-phase reactants, inert or active ionic species or a combination of these techniques to remove unprotected layers of a substrate by chemical and/or physical processes. Unlike wet etching, dry etching is an anisotropic etching process, such that the etch rate may be varied in different directions. One common dry etching technique is called plasma etching, which uses a rf plasma to generate chemically active etchants that form volatile etch species with the substrate. In plasma etching, a wafer is placed between two plates and a voltage is applied to the plates that ionizes gases in between the plates to accomplish the etching. Typically, chlorine or fluorine compounds, most notably CCl4 and CF4, have been tailored for etching SiO2, Si3N4 and metal layers. Another example of dry etching is known as ion etching. Ion etching is accomplished using an inactive species (e.g., Ar ions) either in a beam or with a parallel plate sputtering system.
In a semiconductor device, two different layers may be electrically connected together using a contact, usually made of metal. The fabrication process of the contact depends on the type of layers that are to be connected. Two common techniques used for evaporating metals are physical vapor deposition and chemical vapor deposition, as discussed above.
One common type of semiconductor device is called a transistor. Typically, ICs often comprises many thousands or millions of transistors as building blocks for various electrical circuits. Transistors are comprised of an emitter layer, a collector layer and a base layer. Transistors often act as switches by controlling a primary current flow from the collector to the emitter. A much smaller secondary current is applied to the base to control the primary current.
One type of transistor used in ICs is known as a heterojunction bipolar transistor, or xe2x80x9cHBTxe2x80x9d. HBTs are designed by varying the band-gap energy levels of the emitter and base layers in order to maximize performance. HBTs are used in many applications, such as in semiconductor chips used for communication systems including optical components, wireless power amplifiers for cell phones and base stations, low noise amplifiers and high performance analog to digital converters.
When considering construction of an HBT, the base to emitter separation distance is important for the reliability and high frequency performance of the device. For example, improper production of the base-emitter separation may effect the yield loss of the wafers during the manufacturing process. In addition, the mean time for failure (xe2x80x9cMTTFxe2x80x9d), which is a measure of the reliability of the HBT under typical operating conditions, depends on the base-emitter separation and the properties of the dielectric layer that passivates the base-emitter interface. Further, because a significant part of the base resistance in the HBT is directly proportional to the base-emitter separation, a greater separation provides a higher base resistance, resulting in a degradation of the power gain of the HBT at high frequencies. Accordingly, high frequency performance is improved by providing a base-emitter separation that is as small as possible.
One conventional process of manufacturing a HBT is known as a self-aligned fabrication process. This fabrication process is considered xe2x80x9cself-alignedxe2x80x9d because the base-emitter separation is not defined by lithography. Rather, the base-emitter separation is defined by the shape of the elements in the transistor, specifically, by the shape of an overhang of the emitter over the base as described below.
A conventional HBT self-aligned fabrication process is illustrated in FIGS. 1-4. FIG. 1 shows a cross-section view of a HBT the first stage of the conventional self-aligned fabrication process. As shown in FIG. 1, initially, the transistor comprises and emitter contact 2, an emitter semiconductor layer 4 and a base semiconductor layer 6. The emitter contact 2 is bonded on top of the emitter semiconductor layer 4, which is bonded on top of the base semiconductor layer 6. The emitter contact 2 is typically a metal and provides an electrical connection between the emitter semiconductor layer 4 and any other layers of the semiconductor device.
The next step in the conventional self-aligned HBT fabrication process involves etching away a portion of the emitter semiconductor layer 4 to form an emitter semiconductor mesa 5, as shown in FIG. 2. This etching process is typically accomplished by using a combination of dry and wet etching processes. The wet etching process is stopped so that the emitter contact 2 slightly overhangs the emitter semiconductor mesa 5 (or, the emitter semiconductor mesa 5 undercuts the emitter contact 2).
As shown in FIG. 3, the next step in the conventional self-aligned HBT fabrication process involves the deposition (evaporation) of a base contact 10 onto the base semiconductor layer 6. Typically, the base contact 10 is a metal. In addition, an add-on emitter contact 8 is deposited onto the emitter contact 2. The add-on emitter contact 8 comprises the same material as the emitter contact 2 (i.e., a metal) and provides identical electrical characteristics.
As shown in FIG. 3, the emitter semiconductor mesa 5 and the base contact 10 are separated by a base-emitter separation distance 12. The emitter contact 2 is used as a mask to create the emitter semiconductor mesa 5 during the wet etching process. When the base contact 10 is evaporated around the emitter semiconductor mesa 5 and the add-on emitter contact 8 is evaporated on the emitter semiconductor mesa 5, the overhang of the emitter contact 2 provides a clean separation between the emitter add-on emitter contact 8 (that lands on top of the emitter contact 2), and the base contact 10 (that lands on the base semiconductor layer 6).
Finally, gap 14 must be passivated. Gap 14 is a volume of space between the emitter semiconductor layer 4 and the base contact 10. Gap 14 is generally rectangular in shape, however, other shapes are possible depending upon the size and shape of the emitter contact 2. Typically, the passivation is completed by depositing a layer of silicon nitride or some other polymer layer such as polyimide or Benzo-cyclo-Butene (xe2x80x9cBCBxe2x80x9d).
FIG. 4 shows a top-level view of the HBT during the same stage of the conventional HBT fabrication process as shown in FIG. 3. In FIG. 4, the add-on emitter contact metal 8 overhangs the emitter semiconductor mesa 5, which is encircled by the base contact 10. In addition, the base contact 10 is fabricated within the base semiconductor layer 6.
The conventional HBT self-aligned fabrication process has several inherent drawbacks. For example, it is important to keep the base-emitter separation 12 as small as possible for better performance of the transistor. In the conventional self-aligned fabrication process, the base-emitter separation 12 is determined by use of the overhang of the emitter contact 2 over the emitter semiconductor mesa 5. However, this overhang leads to the creation of gap 14, which is difficult to completely fill in because of the relative closeness of the emitter contact 2 and base contact 10. In particular, because the height of the emitter semiconductor layer 4 is typically in the range of 1000-2500 A, while the height of the base contact 10 is usually in the range of 1000-1500 A, there is only a small opening of approximately 500-1500 A between the emitter contact 2 and the base contact 10. Accordingly voids may be present in gap 14 after passivation, which leads to performance and yield losses.
Another disadvantage in the conventional HBT fabrication process results from the relative closeness of the emitter contact 2 and the base contact 10 (i.e., approximately 500-1500 A). If there is any type of defect on either the emitter contact 2 or the base contact 10, a short circuit may occur between the emitter semiconductor layer 4 and the base semiconductor layer 6 of the transistor, which also may result in unacceptable yield losses.
Another disadvantage of the conventional HBT fabrication process is due to the fact that the under-cut of the emitter semiconductor mesa 5, and relatedly, the base-emitter separation distance 12 is controlled by a wet etching process. Because the wet etching process is relatively difficult to control, especially in the presence of metal which can act as a catalyst (i.e., in this case the emitter contact 2), under some conditions the wet etching process may proceed at a relatively higher rate below the emitter contact 2, thus giving rise to a variation in the emitter semiconductor area (i.e., smaller), the emitter resistance and the base resistance.
Therefore, there is a need for a HBT self-aligned fabrication process and device that provides higher yield and faster performance.
One embodiment of the present invention provides a method of fabricating a semiconductor device including the steps of forming a first semiconductor layer; forming a second semiconductor layer over the first semiconductor layer; forming a mask over a first portion the second semiconductor layer; removing a second portion of the second semiconductor layer not covered by the mask; forming a first electrical connector on the first semiconductor layer; and forming a second electrical connector on the first portion of the second semiconductor layer.
In an embodiment, the first semiconductor layer comprises one or more substances from a group consisting of gallium arsenide and indium phosphide, and is approximately 300-1000 A deep. In an embodiment, the second semiconductor layer comprises one or more substances from a group consisting of silicon, gallium arsenide and indium phosphide and is approximately 1000-3000 A deep. In an embodiment, the mask comprises a photo-resist layer that is approximately 1-3 microns deep.
In an embodiment, the step of forming a mask over a first portion of the second semiconductor layer further includes the step of creating one or more openings in the mask that include a top surface and a bottom surface next to the second semiconductor layer. In addition, the mask is wider at the top surface of the mask and narrower at the bottom surface and is a substantially overhanging shape.
In an embodiment, the step of removing a second portion of the second semiconductor layer is performed by wet etching or dry etching.
In an embodiment, the first electrical connector includes a layer of titanium that is approximately 100-200 A deep, a layer of platinum over the layer of titanium that is approximately 300-400 A deep and a layer of gold over the layer of platinum that is approximately 1000 A-1.2 microns deep. In another embodiment, the first electrical connector includes a layer of palladium that is approximately 100-200 A deep, a layer of platinum over the layer of titanium that is approximately 300-400 A deep and a layer of gold over the layer of platinum that is approximately 1000 A-1.2 microns deep. In an embodiment, the first electrical connector is approximately one micron in width.
In an embodiment, the second electrical connector includes a layer of titanium that is approximately 100-200 A deep, a layer of platinum over the layer of titanium that is approximately 300-400 deep and a layer of gold over the layer of platinum that is approximately 1000 A-1.2 microns deep. In a further embodiment, the second electrical connector is approximately one micron in width.
In an embodiment, the steps of forming the first and second electrical connectors are performed by evaporation. In another embodiment, the semiconductor device is a heterojunction bipolar transistor and the first electrical connector is located approximately 0.3 micron from the first portion of the second semiconductor layer.